A composite video signal contains information which is used by a video system to generate a video picture on a display, monitor or television. Each period, within the horizontal portion of a composite video signal contains information representing one horizontal output line which is to be output on the video display, monitor or television. Each horizontal period includes a horizontal synchronization pulse, a burst signal and a video information signal. In many video transmission systems, color or chrominance information is represented by a particular phase of the chrominance subcarrier signal that is amplitude modulated with color information. The horizontal synchronization pulse is used by a phase locked loop to synchronize the system for displaying the next horizontal line of video information. The burst signal is used to synchronize the phase of the sampling pulses with the phase of the color subcarrier signal. The burst signal has a burst signal frequency equal to 3.58 MHz, which is the frequency of the chrominance subcarrier f.sub.SC. The video information signal then comprises the chrominance subcarrier having different phases amplitude-modulated with chrominance information. The composite color video signal includes both luminance and chrominance information.
A video encoder circuit receives RGB video signals from a video source and encodes a composite video signal representing the RGB signals for transmission to other systems. In order to comply with the National Television Standards Committee (NTSC) video specifications, horizontal synchronizing pulses must be embedded in the video stream in order for the receiving systems to operate properly. These horizontal synchronizing pulses must also be set to exact levels in order to meet the NTSC specifications. A horizontal synchronization pulse is included within each horizontal period of the encoded video signal in order to allow the receiving system to synchronize to each horizontal line of video information within the encoded video signal.
The horizontal synchronization pulse is added into the encoded composite video signal before it is transmitted from the video encoder circuit. Typical prior art video encoder circuits generate exact horizontal synchronization pulse levels by first clamping the encoded video signal to a precise reference voltage level. Accordingly, during the time period when the horizontal synchronization pulse is to be added, the voltage of the composite video signal is exactly known by the system. A switching circuit is then used to switch to a steady, precise horizontal synchronization voltage level, during the horizontal synchronization time period. At the end of the horizontal synchronization time period, the switching circuit switches away from the steady, precise horizontal synchronization voltage level and back to the composite video signal. Accordingly, the resulting encoded video signal includes a horizontal synchronization pulse during the horizontal synchronization time period and the composite video signal during all other periods.
A video encoder circuit, as described above, requires the generation of two precise voltage reference levels for clamping the encoded video signal to the precise reference voltage level and for generating the horizontal synchronization pulse at the horizontal synchronization voltage level. Because of the difficulty associated with generating such precise voltage levels within an integrated circuit, external components are required to generate the necessary voltage levels. A clamping circuit is required to clamp the encoded video signal to the precise reference voltage level. A switching mechanism is also required to switch between the encoded video signal and the horizontal synchronization voltage level at the beginning of the horizontal synchronization pulse and to then switch back from the horizontal synchronization voltage level to the encoded video signal at the completion of the horizontal synchronization pulse. What is needed is a horizontal synchronization pulse generating circuit within a video encoder which does not require the clamping and switching circuitry and the generation of the precision voltage levels required by the video encoder circuits of the prior art.